Compared with conventional utility-scale generation sources, such as fossil-fuel-fired and hydro power plants, the capacity of a DER is relatively small, but the location of DERs can be widely spread across the system. Examples of DERs include diesel engine-generators, wind turbines, photovoltaic units, fuel cells, and energy storages.
The DERs can be conveniently connected to an alternating current (AC) distribution grid. A DER can be used to generate an active power (symbolized as P), a reactive power (symbolized as Q), or combination of both. The reactive power generated by the DER influences the voltages of the grid. Specifically, the amount of reactive power is proportional to the voltages in the grid, i.e., when more reactive power is generated, the voltages in the grid increase, and when less reactive power is generated, the voltages decrease. Without proper regulation, a large number of DERs in distribution systems can cause system-wide violations of the voltage.
The violations of system voltages are usually handled by automatically controlled tap-changing under load transformers, set voltage regulators, and fixed or switched capacitors. However, existing equipment is not inherently designed to handle the variability introduced by DERs, and the lifetime of the equipment could be dramatically reduced due to the increased number of operations that they may perform.
A potential solution to this problem lies on the utilization of DERs to provide reactive power support for voltage regulation with proper control strategies. The effectiveness of system voltage controls through DERs is depended on the control and coordination method used in the power distribution system.
The centralized control and coordination methods are commonly used, see, e.g., U.S. 20130184894 and U.S. Pat. No. 8,401,709. Those methods use a centralized controller to make decisions based on system-wide information, such as bus voltages, and knowledge of all DER units in the systems. The controller issues a command to each distributed resource so that collectively they provide a requested reactive power.
Although the centralized control methods can effectively regulate the system voltages, those methods required a complicated communication network for real-time applications, and any fault or delay in communication at a bus or DER can affect the performance of overall voltage regulation.
Accordingly, there is a need for decentralized control of generation of reactive power in the power distribution systems with the DERs.